An important challenge to the development of an advanced switching mode power supply is to increase the switching frequency of the power switch in the power stage of the power supply, because a power supply operating with higher switching frequency may be designed with smaller volume and less weight. However, higher switching frequency results in more switching loss, and it is therefore required to reduce the switching loss for implementing a high switching frequency design. FIG. 1 shows a conventional quasi-resonance flyback power supply 100, in which a power switch SW is connected in series to a power source Vin and a primary winding P1 of a transformer TX, a capacitor C1 is shunt to the primary winding P1, and a control circuit 102 switches the power switch SW to produce a current on a secondary winding S1 of the transformer TX, so as to charge a capacitor C2 to thereby produce an output voltage Vout.
FIG. 2 is a waveform diagram showing the voltage across the power switch SW of FIG. 1. After the power switch SW is turned off at time t1, a current flows from the secondary winding S1 through a diode D1 to charge the capacitor C2, and the voltage across the power switch SW rises up to a value and remains there until t2. Then the current on the secondary winding S1 becomes off at time t2, but the power switch SW is still off between time t2 and time t3, and the voltage across the power switch SW resonates and thereby has a sinusoidal waveform, due to the oscillation of the magnetizing inductance of the transformer TX and the stray capacitance of the power switch SW and transformer TX. The power switch SW is turned on at time t3, and the voltage across the power switch SW drops off, until the power switch SW is turned off again at time t4. To reduce the switching loss of the power switch SW, the best timing to turn on the power switch SW is when the voltage across the power switch SW is at a minimum, that is, at the valley point of the sinusoidal wave.
Therefore, the key factor of reducing the switching loss is to precisely detect the minimum of the voltage across the power switch SW during the oscillating period. Usually, a differentiator is used to detect the minimum of the voltage across the power switch SW during the oscillating period, for example, proposed by U.S. Pat. No. 6,722,989 to Majid et al. According to the present invention, a valley predicting circuit and method are disclosed for a switching device of a switching mode power supply.